U.S. immigration officials have begun preparing for a major sweep in San Francisco and other Northern California cities in which federal officers would look to arrest more than 1,500 undocumented people while sending a message that immigration policy will be enforced in the sanctuary state, according to a source familiar with the operation.

Officials at Immigration and Customs Enforcement, known as ICE, declined to comment Tuesday on plans for the operation.

The campaign, centered in the Bay Area, could happen within weeks, and is expected to become the biggest enforcement action of its kind under President Trump, said the source, who requested anonymity because the plans have not been made public.

Trump has expressed frustration that sanctuary laws — which seek to protect immigrants and persuade them not to live in the shadows by restricting cooperation between local and federal authorities — get in the way of his goal of tightening immigration.

The operation would go after people who have been identified as targets for deportation, including those who have been served with final deportation orders and those with criminal histories, the source said. The number could tick up if officers come across other undocumented immigrants in the course of their actions and make what are known as collateral arrests.

The sweep would represent the first large-scale effort to target the region since Gov. Jerry Brown in October signed legislation enacting a statewide sanctuary law. Supporters say the law allows undocumented immigrants to cooperate with local police and seek education, health care and other public services without worrying they will expose themselves to possible deportation.

Acting ICE Director Thomas Homan slammed Brown for signing SB54, which he said undermined public safety, and signaled he was prepared to take action.

He said at the time that the federal government would not allow California to be “a sanctuary state for illegal aliens,” and would have no choice but to “conduct at-large arrests in local neighborhoods and at worksites, which will inevitably result in additional collateral arrests, instead of focusing on arrests at jails and prisons where transfers are safer for ICE officers and the community.”

Early this month, Homan told Fox News that “California better hold on tight.” He said that if local politicians “don’t want to protect their communities, then ICE will.”

Told of the planned sweep, California Sen. Dianne Feinstein expressed outrage Tuesday, saying immigrants “must not be targeted in raids solely because they are Californians.” She said a large-scale operation would show that “the administration is carrying out its enforcement actions to make a political point and not based on the security of the country.”

The source who spoke to The Chronicle said the plan calls for immigration officers to be flown in from other parts of the country to help carry out the operation. The sweep could span more than one day and will include enforcement of work sites suspected of illegally employing undocumented immigrants, the source said.

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An ICE spokesman in San Francisco, James Schwab, said in an email Tuesday that the agency “doesn’t comment on future or current operations.”

Liberal-leaning lawmakers in the Bay Area and across California have sparred for years with federal officials over the role local agencies play in immigration enforcement.

The tension intensified after the July 2015 killing of Kate Steinle on a San Francisco Bay pier, when it was revealed that the undocumented immigrant shooter, Jose Ines Garcia Zarate, had been released from San Francisco Jail under the city’s sanctuary ordinance, even though immigration officers had asked that he be turned over for a sixth deportation.

After Garcia Zarate was acquitted of murder charges in November, Homan blamed San Francisco for Steinle’s death, saying the shooting “could have been prevented if San Francisco had simply turned the alien over to ICE, as we requested, instead of releasing him back onto the streets.”

The statewide legislation signed by Brown limits the circumstances under which jails turn over undocumented inmates to the federal government. It also forbids police officers from arresting people on civil immigration warrants and from joining federal agents in task forces intended to enforce immigration laws.

Under the Trump administration, ICE has repeatedly warned that if the agency can’t detain people from local jails, it will be forced to arrest them in the communities that hold such policies.

Last week, a group of politicians, including Reps. Barbara Lee, D-Oakland, and Zoe Lofgren, D-San Jose, sent a letter to Department of Homeland Security Secretary Kirstjen Nielsen requesting a meeting with her and Homan to clarify the remarks he made on Fox News about stepping up enforcement in California.

“The statements are a direct threat to Californians,” the letter read. “These statements are reprehensible and the department’s change in policy will instill fear in our communities. ... Acting Director Homan’s attack on sanctuary cities is not only an infringement of state rights but a direct assault on communities of color.”

In recent years, ICE has not made a practice of conducting major immigration sweeps in cities like San Francisco and Oakland. In September, 27 people were arrested in Santa Clara County as part of an operation targeting sanctuary cities across the country that led to 500 arrests. In June, federal officers arrested 54 people in Central California.

Pratheepan Gulasekaram, a professor at Santa Clara University School of Law and an immigration expert, said that a major sweep in Northern California would echo tactics seen under the George W. Bush administration.

“This is a bluff since California continues to resist,” he said. “This is more show than anything else. They want to make it seem like they are carrying through on this threat. ... I highly doubt that ICE, in the way that it is currently funded, has the ability and resources to maintain something like this on a sustained basis.”

Gulasekaram said the raid would “tear up a lot of lives” but have “little meaningful outcome on public safety.”